The broad objective of this work is to examine the organizational components associated with performance of accountable care organizations (ACOs). With the continued rise in health care costs, policymakers and payers alike have embraced ACOs and other payment reform models as a potential way to improve healthcare and moderate cost growth. Unlike traditional fee-for-service payment models, which pay providers more for providing more care, the payment models underlying accountable care pay networks of providers more for achieving better care at lower costs. Existing evidence from early ACOs shows a modest slowing of spending growth, yet significant variation in savings. This suggests a growing need for rapid yet detailed evaluation to inform the development of the ACO model, particularly during this period of sustained ACO growth. The Affordable Care Act established two voluntary Medicare ACO programs in 2012 with the aims of curbing rising health care costs and improving the quality of care. Although the ACO model continues to grow across public and private sectors, Medicare has become the dominant player in ACO payment reform with 366 Medicare ACOs formed as of January 2014. The Centers for Medicare and Medicaid Services (CMS) had assigned nearly 4 million or 11% of Medicare beneficiaries to an ACO by 2012, and Health and Human Services estimates that ACOs could save Medicare up to $940 million in the first four years. To achieve this promise, research is needed that examines which ACO characteristics are associated with success on both cost and quality metrics. We propose to address this knowledge gap by linking ACO performance data to the National Survey of Accountable Care Organizations (NSACO), a survey that includes data on domains such as ACO contract characteristics, ACO structure and capabilities, and local context. The objective of Aim 1 is to determine identifiers for Medicare ACOs and their component providers. This will allow us to attribute patients to these ACOs using Medicare methodology based on patient care patterns and track the set of physicians associated with an ACO over time. Our second aim is to advance our knowledge of the contributors to successful ACO performance on cost and quality metrics by linking data from the NSACO to Medicare administrative claims data. The linked data will allow us to compare ACO-reported organizational components in the NSACO to components as determined through Medicare claims data, and compare performance statistics as published by Medicare ACO programs to determine the accuracy of the linkage. This work will provide a better understanding of uneven cost savings as reported by CMS among Medicare ACO program participants. It has significant implications for the evolution of payment models.